This invention relates generally to assembling a plate-fin heat exchanger and, in particular, to a technique for tension expanding hairpin tubes into a fin pack unit.
In many small plate-fin heat exchanger units, such as those utilized in room air conditioners, and automotive radiators, hairpin tubes are laced into axially aligned holes formed in the fin plates and tube sheets making up the unit with the open ends of the tubes protruding beyond one of the tube sheets. The walls of the tubes are then expanded radially into contact against the fin collars and tube sheets to hold them in assembly. After expansion, the open ends of the tubes are belled and return bends soldered therein to close the flow circuit of the unit.
As explained in greater detail in U.S. Pat. No. 4,228,573, the tube expansion process is generally carried out by passing tube expanding rods upwardly through the open ends of the hairpin tubes. A backing plate is placed against the tube bends during expansion to prevent the tubes from being driven out of the unit as the tools are forced therethrough. As a result of this holding action the tubes are compressed rearwardly as they are being expanded outwardly by the tools. This in turn, causes the tubes to shrink so that the axial length of each tube can vary dramatically in regard to its neighbor in final assembly. Because of the differences in tube length, belling of the tubes is difficult and generally results in uneven or misaligned bells being formed in the tube ends. The return bend therefore cannot be properly seated within the bells leading to the formation of relatively weak or incomplete solder joints in this critical region.
In order to better facilitate the formation of the tube bells and the joining of the return bends therein, it has been the common practice in the art to bring the open ends of the hairpins a considerable distance out from the adjacent tube sheet. The additional length of tube allows each bell to be brought to full depth without interference from the tube sheet and also permits the return bend joint to be completely immersed in a solder bath to insure that solder will flow into the entire joint region without wetting the tube sheet. The unsupported length of tube between the bell and the tube sheet, however, represents the weakest section in the unit. Hydrostatic tests have shown that the flow circuit will generally rupture in this region when exposed to high internal stresses. Beyond weakening the unit the added length of tubing wastes costly material and thus raises the cost of each unit. Furthermore, the added tube length makes it difficult to compact the unit which in the case of a room air conditioner is of primary importance.